Check /etc/cron.deny first
If there's no /etc/cron.allow in your system, please make sure the user (e.g. oracle) that we would like to authorize it here does not show in /etc/cron.deny.
/etc/cron.allow takes priority over /etc/cron.deny
If you want a positive listing, you can create and add oracle to /etc/cron.allow which will suppress /etc/cron.deny.
Create /etc/cron.allow
[root@test ~]# vi /etc/cron.allow
root
oracle
...Be sure root is in the first place before adding any new users.